1576
Znorg. Chem. 1982, 21, 1576-1581 Contribution from the Department of Chemistry, Faculty of Pharmacy and Biochemistry, University of Zagreb, 4 1000 Zagreb, Croatia, Yugoslavia
Kinetics and Mechanism of Reaction of Cobalt(I1) Protoporphyrin IX Dimethyl Ester with Pyridine and Related Compounds in Predominantly Alcoholic Media in the Presence and Absence of Air's2 ZDRAVKO DOK_UZOVIC,XHAVIT AHMETI,
DUSANKAP A V L O V I ~ , *IVO MURATI,
and SMILJKO ASPERGER*
Received May 20, 1981 Pyridine (py), 3CN-py, 4CN-py, 4CH3-py, piperidine (pip), and imidazole (im) (ligands (L), 104-10-' mol dm-3) added to a freshly prepared alcoholic (methanol, ethanol, 2-propanol plus 2-10% v/v chloroform) solution of cobalt(I1) protoporphyrin IX dimethyl ester (Co"P, 5 X 10" mol dm-3) at 25 "C, in the presence of air, moved the Soret peak of the complex 20 nm bathochromially. This shift is attributed to LConP [L2Con1P]+,via L"Co"P(0,"). The rate of formation of [L2Co"'P]+ increases with increasing concentration of L and of hydrogen ions and decreases when the O2content is lowered. When 0, is completely removed, no [L,CO~~'P]+ is formed. The reaction rate also increases with increased ligand-electron-donating ability and solvent acidity and polarity. The rate constants of the overall reaction of Co"P with a 2000-fold molar excess of 4CH3-py, py, 4CN-py, and 3CN-py in methanol are lo-,, 4 X lo4, and 3 X lo4 s-l, respectively, at 25 "C. The rates in methanol are about 10-20 times faster than in 2-propanol. In aprotic nonpolar solvents, in the presence of oxygen, [L2C0111P]+is not formed. Polar and protic properties of the solvent are important in stabilizing the intermediate LWCouP(O2"). In methanolic solution, in the presence of oxygen, Co"P yields 6C011P(CH30H)(02b). The kinetics of the reaction of this complex (5 X 10" mol dm-3) with py, im, and pip, in methanol, indicates that oxygen is replaced first and methanol second. Limiting rates of oxygen replacement (25 "C) were about 2 X s-I. The corresponding AH* and AS* for im reagent were 61.5 f 2 kJ molF1and -70 f 5 J K-I mol-', respectively. The replacement of methanol takes place at a ligand/complex concentration ratio of about 400, and /cow appears to depend linearly on C,. The second-order rate constants for methanol replacement by py, im, and pip were 250, 100, and 2 s-' mol-' dm3, respectively. The AH*and AS*for im were 75 f 3 kJ mol-l and 30 f 4 J K-l mol-', respectively. A mechanism is discussed.
-
of 02,the formation of [L2C0111P]+proceeds via L6+Co"P(026-) species that are stabilized by the polar and protic properties of the alcoholic solvents. This enables elimination of 02-as HOz. followed by the binding of the amine. If the reaction media do not contain the source of protons, the oxidation, practically, does not take place, and [L2C011*P]+ is not formed. Experimental Section
Several ligands can be easily coordinated in axial positions of Co(II1) porphyrins in a variety of aprotic and mixed proticaprotic solvent^.^,^ On the other hand, binding of amine ligands in the sixth position of Co(I1) porphyrins is very unf a v ~ r a b l e . ~For . ~ example, in toluene, this binding takes place at high ligand concentrations only (with piperidine (pip) a 10s-fold excess over complex concentration is required5). With ligands coordinating less strongly than pip, there is practically no binding to the sixth position at room t e m p e r a t ~ r e .We ~ have studied the kinetics of the reaction of cobalt(I1) protoporphyrin IX dimethyl ester (Co"P, 5 X 10" mol dm-3) with amines as ligands (L: pyridine (py), 3CN-py, 4CN-py, 4CH3-py, and imidazole (im), in concentrations 5 X to 5 X 10-1mol dm-3) at 25 "C. The reactions were carried out in the presence of air, with use of alcoholic solvents that contained 2-1 0% v/v chloroform. Spectrophotometric measurements show that the final reaction product is a hexacoordinated [L2Co"'P]+ complex. The rate of formation of [ L2Co"'P]+ increases with increasing entering-ligand concentration and decreases when the O2 content is lowered. When O2 is completely removed, no [L,Co"'P]+ is formed. The reaction is very strongly proton catalyzed. Furthermore the rates are dependent on basicity and ?r-bonding characteristics of L. This dependence is well established for promotion of oxidation of LCo"P via L6+Co"P(OZ6-)in toluene.'^^ The reaction rates also increase with solvent acidity and polarity. The purpose of this work is to show that, in the presence
Materials. Cobalt(I1) protoporphyrin IX dimethyl ester was prepared from cobalt acetate (Merck, AR grade) and protoporphyrin IX dimethyl ester (Fluka, purum) according to the literature.' The complex was recrystallized from chloroform-methanol and then from benzene. All solvents were Merck A R grade. Absolute methanol was prepared by Lund and Bjerrum's m e t h ~ d . ~Chloroform was washed several times by redistilled water to remove phosgene and other impurities, dried over CaCl,, filtered, and distilled. Pyridine was kept over KOH and freshly distilled before use. Pyridinium chloride was prepared by introducing dry hydrogen chloride into absolute ethanol, at about -15 "C. Cold pyridine was then added, and precipitated pyridinium chloride was filtered off and recrystallized several times from ethanol. Imidazole (Fluka, puriss, p.a.), 4CN-py (Fluka, purum), and 3CN-py (Pliva, purum) were recrystallized several times from ethanol. 4CH3-py (Fluka, purum) was distilled before use. The removal of oxygen from the reaction solution was achieved in the following way: the solution was subjected to sonication at lo5 H z under reduced pressure followed by introduction of oxygen-free argon, reevacuation, and several repetitions of the entire treatment. Spectrophotometry. Absorption spectra were recorded on Cary 16 K and Zeiss DMR 10 spectrophotometers. The sample absorption cell was adapted for work under an oxygen-free atmosphere. Kinetics. The Cary 16 K and Durrum D- 110 stopped-flow spectrophotometers were used for kinetics measurements. The stock solution of Co"P was lo4 mol dm? in chloroform. The reaction solutions were prepared by mixing the stock solution with one of the following alcohols containing the amount of ligand to give 5 X 10" mol dm-3 of complex (and 10% v/v of chloroform): methanol, ethanol, or 2-propanol. In some experiments the reaction solution contained 2% chloroform. Variation of chloroform concentration between 2%
Taken from the thesis submitted by Z. DokuzoviE in partial fulfillment of the requirements for the MSc. degree at the University of Zagreb, 1979, and from a MSc. thesis in preparation by Xh. Ahmeti. Presented in part at the 20th International Conference on Coordination Chemistry, Dec 1979, Calcutta, India; see Abstracts, 11-29, p 293. Kiyoko Yamamoto, Sci. Pup. Inst. Phys. Chem. Res. (Jpn), 71, 111 (1977). N. Datta-Gupta, J. Inorg. Nucl. Chem., 33, 4219 (1972). D. V. Stynes, H. C. Stynes, B. R. James, and J. A. Ibers, J. Am. Chem. SOC.,95, 1796 (1973). D. V. Stynes, H. C. Stynes, B. R. James, and J. A. Ibers, J. Am. Chem. SOC.,94, 1320 (1972). F. Ann Walker, J . Am. Chem. Soc., 95, 1150, 1154 (1973), and references therein. 0020-1669/82/1321-1576$01.25/0 , , I
(8) J. E. Falk, "Porphyrins and Metalioporphyrins", Vol. 2, Elsevier, Am-
sterdam, 1964, p 139. (9) H. Lund and J. Bjerrum, Ber. Dtsch. Chem. Ges., 64, 210 (1931). 0 1982 American Chemical Societv -
Inorganic Chemistry, Vol. 21, No. 4, 1982 1577
Cobalt(I1) Protoporphyrin IX Dimethyl Ester and 10% did not affect absorption spectra and kinetics. The thermastated ahsorption cell (25 f 0.05 "C) was used as the reaction vessel. The kinetics were followed under pseudo-first-order conditions with 10-105-fold molar excess of the amine ligand over the complex. In the presence of air the concentration of oxygen in the reaction mixture was considered as constant, and oxygen was always in large excess (50-60-f0ld'~)over the 5 X lo4 mol dm-3 complex. The kinetics were monitored at the wavelength of the Soret peak of [L2C01"P]+and in some cases additionally at the wavelength of the %ret peak of ConP. The reaction of 6+Co11P(CH30H)(02b)with amine bases was followed by the stopped-flow technique. The solution of the complex (10" mol dm-3 in methanol containing 5% v/v chloroform) was 0.05 "C and mixed with a thermostated thermostated at 25 methanolic solution of amine bases in excess (2 X lo3 to lo5 molar). At this excess the replacement of O2is too fast to be followed, but the replacement of CH30H in [Co"'P(CH30H)(L)]+ formed in the fast step could be easily monitored. Results Absorption Spectra of the Co"P Solutions. The absorption spectra of Co(I1) protoporphyrin IX dimethyl ester in organic solvents are well characterized.' For example, the absorption maxima in the Soret region and of fi and a bands are 404,530, and 563 nm in benzene and 403, 528.5, and 561.5 nm in carbon tetrachloride. We found the following absorption maxima (nm): 401, 528, 562 in chloroform; 402, 528, 558 in methanol; 401, 528, 558 in ethanol; 401, 528, 562 in 2propanol. The solvents have apparently only a small influence on peak positions. On the other hand marked differences exist between the molar absorptivities (e) of solutions in alcoholic and nonalcoholic solvents at Soret and a-band wavelengths. The values of e, at the Soret wavelength, of alcoholic solutions are about 30% below those of nonalcoholic solutions. Likewise the 6 values of a-and @-bandwavelengths of methanolic solutions are 15% and 4%, respectively, smaller than those of nonalcoholic solutions. Corresponding figures for ethanolic solutions are 25% and 22%. The stated differences of molar absorptivities suggest that, in alcoholic solution, a pentacoordinated solvent complex is instantaneously formed, whereas no marked solvent coordinations seem to take place in the nonalcoholic media. This is in agreement with literature data. In fact the addition of a ligand mol dm-3) to a toluene solution of Co"P, in the absence of air, decreased the intensity of both the Soret peak (404 nm) and the a peak (563 nm), with a concomitant small blue shift (8-10 nm) of the latter.s No shift of the Soret peak was o b ~ e r v e d . ~ Spectral Changes during the Reaction Coup + L. The amine ligand, L (about 2000 times the concentration of Co"P), was added to the alcoholic solution of Co"P containing 2% or 10% v/v chloroform, in the presence of air. When py was the entering ligand, the decrease of the Soret peak was accompanied by a simultaneous appearance of five new absorption maxima at 238,244,251,251, and 263 nm. The decrease of Soret absorbance (at 401-404 nm, depending on the alcohol used) and the simultaneous appearance of a new Soret band (at about 420-423 nm, depending on the alcohol) are accompanied by a decrease of the a peak and its bathochromic shift (8-9 nm). In addition, the @/a peak ratio increased due predominantly to the decrease of the a peak. In our opinion the appearance of a new Soret band 20 nm bathochromically to the first is due to the formation of a six-coordinated Co(1II) complex from a five-coordinated Co(I1) complex. This appears to be in agreement with observations of other author^.^^'^^'^
*
'
(IO) The concentration of O2in air-saturatedaqueous solution is 2.7 X lo4 mol dm-' at 25 'C. The solubility of O2 in methanolic solutions is approximately 10% smaller; "Chemical Handbook of Japan", Chemical Society of Japan, Maruzen, Tokyo, Japan, 1973, p 571. (1 1) Reference 6, p 243. (12) N. Datta-Gupta, J . Inorg. Nucl. Chem., 33, 419 (1972). (13) N . Datta-Gupta and T. J. Bardos, J . Pharm. Sci., 57, 300 (1968).
1
.r
0.8
0.12
0.6 0 C
:
9 OL
0.08
0.04
0.2
I
C I
380
LLO
I
I
580
"520
x /nm Figure 1. Spectral changes of a 5 X lo4 mol dm-' solution of Co"P in methanol (10% v/v chloroform). The entering ligand is imidazole in a 100-fold molar excess over the complex; the temperature is 25 "C. Repetitive scans are numbered consecutively at 3-min intervals.
1
3
5
[pyridine] 102/mol dm3
Figure 2. Dependence of the rate of formation of [L,Co"'P]+ from Co"P (5 X lo4 mol dm-3) in the presence of air on pyridine concentration (25 "C) in methanol (0)and ethanol (e),each containing 10% v/v chloroform.
These authors attribute a smaller shift of 16 nm,observed under certain circumstances, to the formation of a six-coordinated Co(I1) complex from a five-coordinated cobalt(I1) complex. Therefore we conclude that under our reaction conditions (alcoholic media, amine ligands, presence of oxygen) the final reaction product is' [L2Co"'P]+. The [L2Co"'P]+ complex, with L = py, was previously prepared and its spectrum recorded (in the visible region only'4). The spectrum shows two maxima, one between 534 and 538 nm and the other between 567 and 570 nm. The small shifts were caused by changes in solvent composition. Our reaction product with py reagent showed the absorption maxima at 534-535 and 568-570 nm, depending on the alcohol used. Almost the same maxima were also found with all other amine ligands. Figure 1 shows the change of absorption spectra of the solution of (14) R. J. McConnel, B. G. Overell, V. Petrov, and B. Sturgeon, J . Pharm. Pharmacol., 5, 179 (1953).
1578 Inorganic Chemistry, Vol. 21, No. 4, 1982 Table I. Rates of Reaction of 5
DokuzoviE et al.
mol dm-3 Co"P with Ligands (Lj in the Presence of Aira
X
l o 2[pip]/mol dm-3
l o 2[py] /mol dm-3
solvent methanolb ethanolb 2-propanolb
1
2
3
4
5
0.5
1
3.0
5
15
1.o 0.2 5 0.47 0.06 4 0.12 0.01 4
2.3 0.5 5 1.05 0.10 4 0.30 0.04 4
4.5 0.9 6 1.20 0.13 4 0.40 0.03 3
9.5 2.0 5 1.25 0.11 4 0.45 0.04 4
13.0 2.0 4 1.30 0.15 4 0.52 0.05 3
1.7 0.3 4 0.30 0.04 3
3.7 0.7 4 0.70 0.08 3 0.100 0.007 4
12.0 2 .o 4 2.2 0.3 3 0.40 0.02 4
3.6 0.4 3 0.600 0.003 4
10.7 1.1 3 1.9 0.1 3
+.
Uncertainties are standard errors of the mean. They were dea L = pyridine, piperidine; temperature 25 t 0.05 "C; product [Co"'PL,] termined from variances of observed rate constants at the respective entering ligands' concentrations (L. Saunders and R. Flemming, "Mathematics and Statistics", The Pharmaceutical Press, London, 1957, p 227). bF'&t row: mean rate of reaction (103k,,,,d/s-' j. Second row: standard errors of the mean. Third row: number of kinetic runs.
Co"P in methanol (containing 10% v/v chloroform) and the im reagent in about 100-fold excess over the complex concentration. It can be seen that the two Soret peaks lie 20 nm from each other. Kinetics of Formation of [Cou1PL2]+from ConP Dependence of the Reaction Rate on Concentration of L. The rate of formation of [L2Co"'P]+ is influenced by the nature of the solvent and ligand. The different shapes of the curves that present these dependences reveal the complexity of the reaction systems. For example, Figure 2 shows that the observed rate constant depends differently on py concentration in methanolic and ethanolic solutions. A similar type of dependence was obtained with pip in the same solvents. The dependence of rate on concentration with some further ligands and solvents (4CH3-py and pip in 2-propanol) is represented by upward bent curves, but the combination of 4CH3-py and methanol gave a downward bent curve. The reaction rates increase with increasing solvent acidity and polarity. For example, reactions of Co"P with py and pip are faster in methanol than in ethanol, whereas in 2-propanol the rates are the lowest; see Table I. Dependence of the Reaction Rate on Basicity of L. The following qualitative order of reactivities was observed with various entering ligands: im > pip > 4CH3-py > py > 4CN-py > 3CN-py. The corresponding pK, values at 25 OC are7,I56.9, 11.1,6.0,5.2,2.2,and2.0.16 Theratewithimisabout 15-fold that with 4CN-py. It appears that the ligand basicity (o-donor ability) is of primary importance. The exception is imidazole, which reacts faster than expected from its basicity. Since ligands coordinated trans to the oxygen will compete for Aelectron density on the cobalt, the binding of O2to LCo"P will be sensitive to the *-donating or -accepting ability of the axial ligand L. As pointed out,5 good A acceptors will decrease *-electron density on the metal (weaker C0-O2 bond), while good A donors will promote bonding of oxygen. Since imidazole is a much better A donor than, e.g., ~ y r i d i n e the , ~ high rate with imidazole can be understood. The additional explanation could be that pip reacts somewhat slower than expected from its a-donor ability. The slower reaction of pip is probably due to van der Waals repulsions between the A system of the porphyrin and piperidine hydrogen atoms at the ring positions nearest to the nit r ~ g e n .It~ should be noted that this dependence of the reaction rate on ligand basicity is analogous to that established in oxidation of LCo"P in toluene, via *+LCO~~P(O~"), probably to p-peroxo-bridged Co(II1) dimer.5 (1 5) R. C. Weast, Ed., "Handbook of Chemistry and Physics", 53rd ed.,The Chemical Rubber Co., Cleveland, OH, 1972-1973,p D-117. (16) Value determined in our laboratory by pH measurements at equal concentrationsof 3CN-py and 3CN-pyH+ in aqueous solution at 25 "C
in argon atmosphere.
Table 11. Rates of Reaction of 5 x mol dm-3 CoIIP with Pyridinium Ion, in the Presence of Aira 1O4 [py.HCl]/mol dm" solvent
1
3
4.5
7
10.0
methanolb
4.5 0.2 3 1.50 0.07 3 1.9 0.1 3
8.0 0.5 3 4.0 0.2 4 4.7 0.3 4
10.5 0.5 3 5.7 0.3 3 7.0 0.4 4
14.7 0.8 4 8.8 0.5 3 10.6 0.6 3
19.8 1.0 3 12.3 0.7 3 14.7 0.8 3
ethanol 2-propanolb
a Temperature 25 ? 0.05 "C, product [Co"'P(py j1 ] +. First row: mean rate of reaction ( 1 0 3 k 0 b , ~ ~ - 1 )Second . row: standard errors of the mean. Third row: number of kinetic runs.
Dependence of the Reaction Rate on Acidity of the Solution. The addition of acid (hydrochloric, perchloric, acetic, and p-toluenesulfonic acids were used) accelerated the reaction markedly. For example, addition of 4 X mol dm-3 p toluenesulfonic acid to the reaction mixture containing 5 X 10" mol dm-3 Co"P and 3 X mol dm-3 py in methanol increased koW from 7 X lo4 to 0.1 s-l. The addition of acid reduces the concentration of pyridine, forming pyridinium ion. Under the above concentration conditions pyridine was still in more than 100-fold molar excess over complex concentration. The reaction of Co"P with pyH+ in the presence of oxygen, yielding [ C ~ ~ ~ ' P ( p y ) ,was l + , also carried out. It was found that rates with pyH+ are about 100 times as high as those with py. For example, when concentrations of both pyH+ and py are 0,001 mol dm-3, the observed rate constants in methanol, s-l, at 25 "C, are (1.98 f 0.1) X and (2.2 f 0.2) X respectively. Data in Tables I and I1 reveal that koM for pyH+ and py are of the same order of magnitude, although concentrations of pyH+ used were 100 times lower than those of py. It should also be noted that with pyH+ reagent the influence of solvent media is much less pronounced than with py reagent (compare Tables I and 11). This observation allows the conclusion that acidity of the media is more important in determining the reaction rate than solvent polarity. Dependence of the Reaction Rate on Oxygen Concentration. The rate of formation of [L2Co"'P]+ decreases with decrease of oxygen concentration. When oxygen is completely removed, there is no formation of [L2Co"'P]+. For example, the absorption spectrum of the reaction mixture containing 5 X 10" mol dm-3 Co"P and 3 X mol dm-3 py in methanol in an oxygen-free atmosphere remained unchanged for several weeks at room temperature. When the reaction vessel was opened to air, the spectrum changed immediately, indicating the formation of [L2Co11'P]+. Hydrogen and nitrogen were used
Inorganic Chemistry, Vol. 21, No. 4, 1982 1579
Cobalt(I1) Protoporphyrin IX Dimethyl Ester Table 111. Rate Constant of Formation of [Co"'PL,] Various Temperaturef
+
at
12
0.001 0.1 0.001
py
im
2.1 10.1 8.3
2.2 10.0 8.1
1.3
1.8 10.8 5 .O
i
mol d ~ n -L~=; pyridine b y ) , imidazole (im). a Co'IP, 5 X Solvent: methanol containing 10% v/v chloroform. Measurements made in presence of air. e,.
10
i
380
LLO
500
30
Figure 4. Dependence of the rate constant, k ~ b s d ( ~ ) - ( ' for ) , the replacement of oxygen by pip in 5 X 10" mol dm-, *+Co"P(CH30H)(02&)(0)and of the rate constant koW(l0) for the replacement of CH,OH by pip in [Co"'P(CH,OH)L]+ (@) on piperidine concentration (solvent MeOH (5% v/v CHCI,), presence of air, 25 "C).
d
I
20
lO'Ipipl /mol dm3
5 60
h/nm
Figure 3. Spectral changes of a 5 X 10" mol dm-, solution of Co"P in ethanol (containing 10%v/v chloroform) in the presence of air (kM E 2 X lo4 s-' at 25 OC).
for lowering the oxygen content, and oxygen-free argon was used for complete removal of oxygen (see Experimental Section). Effect of Temperature on Reaction Rates. Data are given in Table I11 for entering ligands py and im. It can be seen that the reaction rate is very little influenced by temperature. This is obviously due to complexity of the reaction which involves exothermic preequilibria, as is binding of L and O2 to Co"P (see Discussion). Reaction of ConP with Molecular Oxygen in Methanol and Ethanol The alcoholic solutions of Co"P, in the absence of oxygen, are stable for weeks. No spectral evidence to indicate the formation of hexacoordinated species was found in an oxygen-free atmosphere. On the other hand, if alcoholic solutions are exposed to air, slow spectral changes are observed. In methanol the Soret peak shifts from 402 to 418 nm and in ethanol from 401 to 418 nm, with simultaneous increase of the /3/a peak ratio (Figure 3). After 24 h there are practically no more spectral changes. The starting Soret peaks at 402 and 401 nm disappeared, suggesting that Co"P is practically no more present in the solution. Upon bubbling of oxygen-free argon through the solution, the initial absorption spectrum was restored. We attribute the observed spectral changes to equilibrium 1. The 16-17-nm bathochromic shift of the Soret Co"P(CH,OH)
+ O2 + *+Co"P(CH,OH)(O,")
(1)
1O[L1 / m o l dm3
Figure 5. Dependence of the rate of replacement of CH30H in 5 X 10" mol dm" [Co"'P(CH,OH)L]+ by L (pyridine, imidazole, and piperidine) on concentration of L in MeOH ( 5 % v/v CHCI,) at 25 OC.
peak indicates the formation of a Co"P hexacoordinated specie^.^ When the alcohol is in the fifth position, electron transfer from cobalt to oxygen is most probably very small. It is claimed to be small even when amine ligands are in the fifth position, as found in nitrile The formation of a p-peroxo complex is unlikely, since this would, most probably, cause a 24-nm shift of the Soret peak toward longer wavelength^.^ Moreover, the reversibility of reaction 1 speaks against p-peroxo complex formation. Kinetics of Reaction of 6+ConP(CH30H)(02a-) with Amine Ligands The reaction of *+Co"P(CH,OH)(O,") with amine ligands in alcoholic media, yielding [Co111PL2]+,proceeds in two steps, as can be inferred from Figure 4. Oxygen appears to be replaced first, with simultaneous oxidation of Co(I1) to Co(111), and methanol is replaced second (see Discussion). The concentration of starting complex, *fC~11P(CH30H)(02b), was 5 X lod mol dm-3. Up to a 200-fold molar excess of the entering ligand over the complex concentration, the rate of the second step can be neglected and the first step easily studied. The rates of the second step were measured at lo3to ( 5 X lo4)-fold molar excess of the entering ligand, with use of stopped-flow techniques (Figure 5). The kinetics of both steps in methanol were monitored by measuring the absorbance
1580 Inorganic Chemistry, Vol. 21, No. 4, 1982
Dokuzovie et al.
at 422,422.5,and 421 nm for py, im, and pip entering ligands, respectively. At these wavelengths were the absorption maxima of both ligand0 methanolo and diligando reaction products. The measurements were possible because the molar absorptivities of the diligando final reaction product at these wavelengths are larger than those of the monoligando intermediate. Most of the kinetics was in the stopped-flow range. The second-order rate constants for the replacement of C H 3 0 H by py, im, and pip are 250,100,and 2 s-l mol-' dm3 at 25 OC. The dependence of koW on replacement of oxygen and methanol by pip is shown in Figure 4. Analogous curves are obtained for py and im. The limiting rates for replacement of oxygen, obtained with these ligands, were 2 X s-l at 25 OC. Figure 5 shows that koM for the replacement of C H 3 0 H by L (py, im, pip) depends linearly on the concentration of L up to about 0.2 mol dm-3. Activation Parameters. The rate constants for the replacement of oxygen by im (at Ch = 9.2 X mol dm-3 and * + C O ~ ~ P ( C H ~ O H ) (concentration O,~) = 5 X 10" mol dm-3) were determined at 18,21,25,and 30 OC as 4.4,6.0,8.9, and 12.5 X s-', respectively. The Arrhenius plot gave a good straight line. From the least-squares method, AH* and AS* were found to be 61.5f 2 kJ mol-' and -70 f 5 J K-'mol-', respectively. The rate constants for the replacement of C H 3 0 H by im (8.84X lo4 mol dm-3) were determined at 18.5,21.5,25.4, 28.0,and 31.0OC as 2.8,3.9, 5.5,8.7,and 9.9 s-l, respectively. AH* and AS*were 75 i 3 kJ mol-' and 30 f 4 J K-' mol-', respectively. Discussion We have shown that the reaction Co"P + 2L yielding [L2?"'P]+, in the presence of oxygen, can be readily carried out in alcoholic media. It was also shown that the reaction is acid catalyzed and that the rate is increased by increase of solvent polarity. The importance of solvent polarity in stabilization of the polar L6+Co11P(026-)species was previously recognized." On the other hand it is important to note that the reaction mentioned above does not take place in dry toluenes but, we found, does in toluene containing about 0.01% water. However, in this case the reaction is a few hundred times slower than in methanol. It is reasonable to assume that 0.01% water in toluene did not significantly change the solvent polarity. Probably the effect of water (which was in about 1 04-fold molar excess over the 5 X 10" molar Co"P complex) is caused by hydrogen bonding to the L6+Coi1P(Oz") species.'*J9 This leads to a stabilization of L6+Co"P(OZb). The reaction probably proceeds by steps 2-5. We assume the existence of equilibrium 2 because of the quite large (30%) K
+
Co"P ROH (R0H)Co"P (2) decrease in the molar extinction coefficient of the Soret peak of Co"P when the solvent is changed from chloroform to alcohol. Addition of amine ligands L leads to formation of new equilibria. Alcohol coordinated in the axial (fifth) position is replaced by L (eq 3). The position of equilibrium 3 depends ( R O H ) C ~ I I P+ L
2
+ROH
(3) on u- and .rr-bonding characteristics of L. If the reaction solution is exposed to air, equilibrium 4 is quickly established. LC0"P
LCOIIP
s
+ o2
L6+C0"P(O2b)
(4)
(17) H. C. Stynes and J. A. Ibers, J. Am. Chem. Soc., 94, 5125 (1972). (18) The possibility of hydrogen bonding of imidazole to L'Co1JP(026) was
recognized by D. V.Stynes, J. A. Ibers, and B. R. James, J . Am. Chem.
SOC.,95, 1142 (1973). (19) R. S. Drago, J. P. Cannady, and K. A. Leslie, J . Am. Chem. SOC.,102, 6014 (1980).
%
lo2[imidozolel / mol
dm.'
Figure 6. Dependence of reaction rate of 5 X lo4 mol dm-3 Co'IP with imidazole on imidazole concentration. Solvent: ethanol (25 "C), in presence of air. Absorbance increase at 424 nm (formation of [Co"'P(im),]+ ( 0 ) )and decrease at 401 nm (disappearance of Co"P (0))were monitored.
As already mentioned the coordination of L in the sixth position of Co" is very u n f a v ~ r a b l e . ~On * ~the other hand O2 can c ~ o r d i n a t e , ~ . with ' . ~ ~the ~ ~ tendency of accepting ( z 2 / r * ) , and ( x z , K*),. electrons, which leads to partial oxidation of cobalt, giving the oxygen superoxide c h a r a ~ t e r . It~ ~ was ~~~ also pointed outz3that there is a wide variation in the amount of electron transfer to Oz, which depends on the nature of ligands coordinated to cobalt. The extent of electron transfer depends also on solvent acidity and solvent polarity. The rate-determining step is, most probably, reaction 5. The fact L6+C0"P(O?-)
+L
H+
[L~CO"'P]++ HOy
(5)
that the overall reaction rate is strongly accelerated by hydrogen ions supports the above claim. The HOz. radical might initiate subsequent radical reactions involving HOT and HO., ending upzbz8 with HO-. Large standard errors of some of our kinetic results (up to 20%; see Table I) might be partly due to radical-induced oxidations of Co(I1). In the overall reaction, eq 2-5, there seems to be no accumulation of reaction intermediates. This can be inferred (a) from the existence of isosbestic points in Figure 1 and (b) from the fact that rates of disappearance of Co"P and rates of formation of [Co"'PL,]+ in the overall reaction are equal (see Figure 6 ) . In the overall reaction (2)-(5), unfortunately, we could not separate reaction 5 from its preequilibria (2)-(4) and study the kinetics of O2 replacement in L6+Co1IP(O:-). But, as already described, we prepared, in the solution, 6+Co11P(CH30H)(026) and were able to study the kinetics of O2 replacement by L in this complex. The fact that practically (20) D. V. Stynes, H. C. Stynes, J. A. Ibers, and B. R. James, J . Am. Chem. SOC.,95, 1142 (1973); J. A. Ibers, D. V. Stynes, H. C. Stynes, and B. R. James, ibid., 96, 1358 (1974). (21) (a) F. Ann Walker, D. Beroiz, and K. M. Kadish, J . Am. Chem. SOC., 98, 1484 (1976); (b) F. Ann Walker, ibid., 92, 4234 (1970). (22) F. Basolo, 8. M. Hoffman, and J. A. Ibers, Acc. Chem. Res., 8, 384 (1975). (23) R. S. Drago, T. Beugelsdijk, J. A. Breese, and J. P. Cannady, J . Am. Chem. SOC.,100, 5374 (1978), and references therein. (24) B. S. Tovrog, D. J. Kitko, and R. S. Drago, J. Am. Chem. Soc., 98,5144 (1976). (25) R. Dreos, G. Tauzher, G. Costa, and M. Green,J . Chem. SOC.,Dalton Trans., 2329 (1975). (26) J. Weiss, Naturwissenschaften, 23, 64 (1935). (27) W. G. Barb, J. H. Baxendak, P. George, and K. R. Hargrave, Trans. Faraday SOC.,47, 462 (1951). A, 2044 (1969). (28) S. Agperger, I. Murati, and D. PavloviE, J. Chem. SOC.
Inorg. Chem. 1982. 21. 1581-1586 limiting reaction rates were obtained for the first step (oxygen replacement, Figure 4) is consistent with the following mechanism: 6+Co11P(CH30H)(02")+ HL+
k
2 k-i
[Co1"P(CH30H)(02H)]+
+L
(6)
k2
[CO"'P(CH~OH)(O~H)]+ +L [CO"'P(CH~OH)L]+ HOy ( 7 ) +
+
A similar mechanism was discussed previously25for oxidation of a cobalt(I1) dioxatetraazaboracyclotetradecatetraene complex. The application of the steady-state approximation to the short-living intermediate [Co"'P(CH30H)(02H)]+ leads to eq 8 for the If the only source of protons is
methanol, then eq 9 can be used instead of eq 8, where K = (9)
[HLf] [CH30-]/ [L]. Equation 9 explains well the quasi-limiting reaction rates in replacement of oxygen by L (Figure 4, open circles). The second step (Figure 4,full circles, and Figure 5) is the replacement of the coordinated solvent molecule, eq 10. If
+
[CO'~'P(CH,OH)L]+ L
k,(lO)
[Co"'PL2]+
+ CH30H (10)
1581
in *+Co"P(CH3OH)(O2") methanol were replaced by L before oxygen, no limiting rates would have been obtained, since this would be a replacement of a methanol molecule by L in methanol as the solvent. Such a replacement must obey the second-order rate law.29 Indeed, this is true for the second step, eq 10, which exhibits linear dependence of koM(lO) on L concentration (Figure 4,full circles, and Figure 5). The negative entropy of activation (A,!?* = -70 J K-' mol-') of the first step, eq 6 and 7 , where (6) is considered to be the rate-determining step, is in accordance with the associative nature of this reaction. On the other hand the positive entropy of activation (30 J K-l mol-') of the second step, eq 10, is to be expected for a dissociative-type process. It is interesting to note that the rate of the overall reaction (2)-(5), as already discussed, increases with increased ligand-electron-donating ability. On the other hand, the order of reactivity in reaction 10 is reversed. In the overall reaction the entering ligand was also a directing ligand for subsequent reactions, which appears to be a dominating feature, whereas in reaction 10 this is not so. The relatively slow reaction with pip is, most probably, due to steric effects.' Acknowledgment. Thanks are due to Professors H. Hoffmann and H. L. Krauss, University of Bayreuth, Bayreuth, West Germany, who provided facilities for spectral measurements in an oxygen-free atmosphere. Registry NO.CO"P, 14932-10-6;Co"P(CH,OH)(02), 80285-29-6; py, 110-86-1; pip, 110-89-4; im, 288-32-4; 3CN-py, 100-54-9; 4CN-py, 100-48-1; 4CH3-py, 108-89-4; pyridinium, 16969-45-2. (29) D. PavloviE, D. h i e , and S. ASperger, J . Chem. Soc., Dalton Trans., 2406 (1976).
Contribution from the Laboratoire d'Electrochimie et de Chimie Physique de Corps Solide, ERA au C N R S No. 468, Institute de Chimie, Universite Louis Pasteur, 67000 Strasbourg, France, the Laboratoire Associe au C N R S No. 31, Institut de Chimie, Universite Louis Pasteur, 67008 Strasbourg Cedex, France, the Department of Energy and Environment, Brookhaven National Laboratory, Upton, New York 11973, and the Department of Chemistry, University of Houston, Houston, Texas 77004
Reduction of Copper Tetracyanotetraphenylporphyrin in Nonaqueous Media. Formation of Copper(1) A. GIRAUDEAU,la A. LOUATI,Ia M. GROSS,*la H . J. CALLOT,Ib L. K. HANSON,*" R. K. RHODES,Id and K. M. KADISH*ldsC
Received August 21, 1981 The total electrochemical reduction of copper tetracyanotetraphenylporphyrin, (CN),(TPP)Cu, was investigated in dimethylformamide (DMF) by the techniques of polarography, cyclic voltammetry, thin-layer spectroelectrochemistry, ESR spectroscopy, and extended Hiickel calculations. The ESR results establish the existence of a Cu(I1) dianion. The electrochemical reactions of (TPP)Cu and (CN),(TPP)Cu are compared, and the differences in redox behavior can be rationalized by the calculations. Unlike (TF'P)Cu, which may be reduced by a total of six electrons in four steps, (CN),(TPP)Cu is reduced by seven electrons in five steps. This new wave occurs at El,2 = -1.95 V in DMF and is attributed to reduction of the Cu(I1) dianion to yield a Cu(1) complex. Formation of this species is reversible on the cyclic voltammetry time scale and an electronic spectrum has been obtained. This spectrum consists of a split Soret band at 390.0 and 480.4 nm and a strong band in the visible at 688.2 nm.
Introduction During the last 15 years a large number of studies have reported oxidation and reduction potentials for metalloporphyrin c o m p l e x e ~ . ~In~ ~nonaqueous media monomeric (1) (a) Laboratoire d'Electrochimieet de Chimie Physique de Corps Solide, Universite Louis Pasteur. (b) Laboratoire Associe au CNRS No. 31, Institut de Chimie, Universite Louis Pasteur. (c) Brookhaven National Laboratory. (d) University of Houston. (e) On leave at the Universite Louis Pasteur, 1980-1981. 0020-1669/82/ 1321-1581$01.25/0
metalloporphyrins are reduced (usually reversibly) by at least two single electron transfer steps to yield 7r anions and dianions. This is true for complexes with over 30 different central metals, including copper, and is independent of the charge on the central metal, which may vary from 1+ (in the case of cobalt (2) D. Dolphin, Ed., 'The Porphyrins", Vol. V, Academic Press, New York,
1978. (3) K. M. Smith, Ed., 'Porphyrins and Metalloporphyrins", Elsevier,
London, 1975.
0 1982 American Chemical Society
